The invention relates to a circuit arrangement for the indication of loop closure during the ringing condition for a subscriber circuit which is connected to the exchange of a digital time multiplex telephone system by way of a subscriber termination circuit comprising a subscriber line interface circuit which, in part, provides the ringing supply and signalling, and a signal processing unit which, in part, provides for analog to digital conversion and digital to analog conversion as well as for filtering.
Particular requirements concerning sensitivity and response time are placed on the indication of loop closure, since the ringing current which is superimposed on the supply circuit when the subscriber loop is closed, flows through the telephone set microphone and can cause destruction if the time interval is too long before ringing is disconnected.
For the indication of ringing, a proportional voltage has in the past been derived from the subscriber termination line current, from which the alternating component arising from the ringing signal was filtered out by a low-pass filter. A comparison was made against a fixed preset threshold value which, when exceeded, resulted in a corresponding signal which was evaluated in a group processor, that is, a processor allocated to a plurality of subscriber termination circuits.
A basic task of the invention is to reduce the hardware cost of such a ringing indication circuit and, at the same time, to be as flexible as possible with regard to the response threshold and the response time.
In accordance with the invention, this task is accomplished in a circuit arrangement of the type referred to at the outset in that this circuit arrangement includes a changeover switching arrangement, by way of which, a voltage signal proportional to the subscriber termination line current is applied to the signal processing unit during the ringing condition, in place of the speech signal present during the speech condition and in that this voltage signal is subjected to an analog to digital conversion and filtering as for the speech signal at other times except that the high-pass filtering otherwise employed is avoided, and in that the resulting digital signal is further therein summed over at least one period of the ringing signal frequency and as a sum signal is compared with a reference value which provides a loop closure signal when the reference value is exceeded by the sum signal.
Accordingly the idea of rendering the capabilities of the signal processing unit useful for the ringing indication is basic to the invention. Since the requirements for this comprise, for the most part, a corresponding matching of processing software in return for which the previously mentioned hardware components for ringing indication are not needed, the hardware costs are lower overall for the circuit arrangement in accordance with the invention.
Further arrangements of the invention concern embodiments relating to the summing and threshold determination of the digital signal derived from the loop current during the ringing phase which make it possible to match the concept according to the invention optimally to practical conditions, namely, to the samples and to the switching behavior on switching on the ringing signal which are basic to the communication system with which the circuit arrangement in accordance with the present invention is associated.
A further embodiment of the invention concerns the special case wherein the ringing signals are applied to the subscriber termination circuitry as digital signals or else are generated in the signal processing unit as digital signals, are therein subject to digital to analog conversion and are applied to the line amplifier circuit of the subscriber termination circuit, and furthermore, amplification specifically for the ringing signal at the input state of this amplifier circuit and a corresponding bias voltage at the output stage thereof are enabled.
The loop closure signal resulting from the comparison in this case is utilized for further switching functions besides interrupting the application or, as the case maybe, the generation of the digital ringing signals.